Systems and methods for membraneless dialysis

ABSTRACT

Devices, systems and methods are disclosed for removing undesirable materials from a sample fluid by contact with a second fluid. The sample fluid flows as a thin layer adjacent to, or between, concurrently flowing layers of the second fluid, without an intervening membrane. In various embodiments, a secondary separator is used to restrict the removal of desirable substances and effect the removal of undesirable substances from blood. The embodiments may be used for the removal of components from a sample fluid that vary in size. When blood is the sample fluid, for example, this may include the removal of small molecules, middle molecules, macromolecules, macromolecular aggregates, and cells, from the blood sample to the extractor fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/499,038, filed Jul. 7, 2009, which is a division of U.S. applicationSer. No. 11/776,360, filed Jul. 11, 2007, now U.S. Pat. No. 7,588,550,which is a division of U.S. application Ser. No. 10/801,366, filed Mar.15, 2004, abandoned, which claims the benefit of U.S. ProvisionalApplication No. 60/454,579, filed Mar. 14, 2003, expired, all of whichare hereby incorporated by reference herein in their entireties.

FIELD

Generally speaking, the present invention relates to the purification ofa sample fluid. More particularly, the present invention relates to thepurification of a sample fluid, blood fluid) by selectively removingcomponents using a microfluidic membraneless exchange device.

BACKGROUND

Extracorporeal processing of blood is known to have many uses. Suchprocessing may be used, for example, to provide treatment of a disease.Hemodialysis is the most commonly employed form of extracorporealprocessing for this purpose. Additional uses for extracorporealprocessing include extracting blood components useful in either treatingothers or in research. Apheresis of plasma (i.e., plasmaphesis) andthrombocytes, or platelets, are the procedures most commonly employedfor this purpose.

Many different extracorporeal blood processing processes have beendeveloped, each of which seeks to remove certain components from theblood, depending on the reason for processing the blood. (It will beunderstood that as used herein, blood, or blood fluid, refers to anyfluid having blood components from which extraction of certaincomponents, such as toxins or albumin, is desired.) The most commonprocess utilizes an artificial membrane of substantial area, acrosswhich selected blood components are induced to flow. This flow isgenerally induced by a transmembrane difference in either concentrationor pressure, or a combination of the two. Another form of bloodprocessing calls for the separation of certain components from blood bypassing the blood over sorbent particles. In yet other forms of bloodprocessing, not practiced as commonly, blood is directly contacted withan immiscible liquid (e.g., a fluorocarbon liquid), with the desiredresult being the removal of dissolved carbon dioxide and the provisionof oxygen. The usefulness of blood processing techniques employingimmiscible liquids is limited, however, because these immiscible liquidsgenerally have very limited capacity to accept the blood components thatit is desirable to extract.

One common example of a therapeutic use for blood processing is themitigation of the species and volume imbalances accompanying end-stagerenal disease. The population of patients treated in this manner (i.e.,through hemodialysis) exceeds 260,000 and continues to grow, with thecost of basic therapy exceeding $5 billion per year excludingcomplications. The overwhelming majority of these patients (about 90%),moreover, are treated in dialysis centers, generally in thrice-weeklysessions. While procedures have been—and continue to be—refined, thecomponents and the geometry of hemodialysis were largely fixed in the1970's: a bundle of several thousand, permeable hollow fibers, eachabout 25 cm long and about 200 μm internal diameter, perfused externallyby dialyzing solution, with the device operated principally in adiffusive mode but with a transmembrane pressure applied to induce aconvective outflow of water. Upward of 120 liters per week of patientblood are dialyzed against upwards of 200 liters per week of dialyzingsolution, often in three weekly treatments that total as little as sevento nine hours per week. These numbers vary somewhat, and competingtechnologies exist, but the basic approach just described predominates.

Despite the benefits of therapies (e.g., hemodialysis) using the variousforms of blood processing described above, the prolongation of lifeachieved is complicated by the progression and complexity of the diseasethe therapies are used to treat (few patients on dialysis are evercompletely rehabilitated), and by several problems that are innate tothe therapies themselves. For example, problems arise with bloodprocessing as a result of the contact of blood with extensive areas ofartificial membrane (as in the case of hemodialysis), and well as thecontact of blood with sorbents or immiscible fluids as described above.In particular, this contact often induces biochemical reactions in theblood being processed, including the reactions that are responsible forclotting, activation of the complement systems, and irreversibleaggregation of blood proteins and cells.

Another problem associated with known blood processing techniques isthat the contact of blood with an artificial membrane (or anothermedium, such as a sorbent or immiscible fluid) is likely to cause theblood-medium interface to become fouled. It is generally known thattherapeutic interventions (e.g., those related to end-stage renaldisease) are optimally conducted with slow delivery and in as nearly acontinuous fashion as possible, in emulation of the continuous action ofa natural kidney. However, fouling caused by the contact of blood withthe medium limits the time that a device which contains these interfacescan be usefully employed. As a result, portable blood processing devicesbecome impractical, and patients are generally forced to undergo thetype of episodic dialysis schedule described above, which creates manynegative side effects such as physical exhaustion and excessive thirst.Moreover, even while daily dialysis (e.g., 1.5-2.0 hours, six days perweek) or nocturnal dialysis (e.g., 8-10 hours, 6-7 nights per week)improves this situation by extending treatment times, a patient usingone of these forms of treatment is still required to remain near ahospital or clinical facility that can administer the dialysisprocedure.

In light of the above, it would be desirable to provide techniques forprocessing blood in which treatment times are extended (withconsequently lower rates of flow) and that do not require a patient toremain near a hospital or clinic. Moreover, it would also be desirableto provide techniques for processing blood that eliminate (or at leastreduce) the inducement of undesirable biochemical reactions, and wherethe blood-medium interfaces do not become fouled.

SUMMARY

The above and other deficiencies associated with existing bloodprocessing processes are overcome in accordance with the principles ofthe present invention which are described below. According to one aspectof the invention, a membraneless exchange device for extractingcomponents from a sample fluid is described which includes first, secondand third inlet channels, first, second and third exit channels and amicrofluidic extraction channel connected to the first, second and thirdinlet channels and the first, second and third exit channels. Moreover,laminar flows of a first extractor fluid, the sample fluid, and a secondextractor fluid are established inside the extraction channel, andsheathing of the sample fluid by the first and second extractor fluidssubstantially limits contact between the sample fluid and the surfacesof the extraction channel.

According to another embodiment of the present invention, a system forperforming hemodialysis is provided which includes a membranelessexchange device including first and second dialysate inlet channels,blood inlet and exit channels, first and second dialysate exit channelsand a microfluidic dialysis channel connected to the first and seconddialysate inlet and outlet channels and the blood inlet and exitchannels. Moreover, laminar flows of a first dialysate fluid, bloodfluid, and a second dialysate fluid are established in order inside thedialysis channel, and at least some of the components of the blood fluidexits the device through the first and second dialysate exit channels.Additionally, according to the invention, a secondary processor receivesthe dialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels.

In yet another embodiment of the present invention, a method forextracting components from a sample fluid is provides which includesestablishing laminar flows of a first extractor fluid, the sample fluidand a second extractor fluid inside a microfluidic extraction channel.Sheathing of the sample fluid by the first and second extractor fluids,moreover, substantially limits contact between the sample fluid and thesurfaces of the extraction channel. The method further includeswithdrawing the first extractor fluid, the sample fluid and the secondextractor fluid from the extraction channel such that at least a portionof the sample fluid is removed together with the first extractor fluidand the second extractor fluid and apart from the remainder of thesample fluid.

A method for performing hemodialysis is also provided which includesestablishing laminar flows of a first dialysate fluid, blood fluid and asecond dialysate fluid inside a microfluidic extraction channel,withdrawing the first dialysate fluid, the blood fluid and the seconddialysate fluid from the extraction channel such that at least some ofthe components of the blood fluid are removed together with the firstdialysate fluid and the second dialysate fluid and apart from theremainder of the blood fluid, and providing the first and seconddialysate fluids and the at least some of the components of the bloodfluid to a secondary processor.

In general, however, the present invention is directed towardmicrofluidic membraneless exchange devices and systems, and methods ofmaking the same, for selectively removing undesirable materials from asample fluid (e.g., blood fluid) by contact with a miscible fluid(extractor fluid or secondary fluid, e.g., dialysate). A microfluidicdevice, as considered in this application, has channels whose height isless than about 0.6 mm, where “height” is the dimension perpendicular tothe direction of flow and also perpendicular to the interfacial areaacross which transport occurs. For example, flow patterns and speciesexchanges occur when blood is flowed as a thin layer adjacent to, orbetween, concurrently flowing layers of a secondary fluid, without anintervening membrane. The secondary fluid, moreover, is generallymiscible with blood and diffusive and convective transport of allcomponents is expected. The following reference which refers tomembraneless devices described below is hereby incorporated by referencein its entirety: Leonard et al., Dialysis without Membranes: How andWhy?, Blood Purification 22 (1) 2004 92-100.

Sheathing a core of blood with the miscible fluid, or assuring that themiscible fluid lies between at least a substantial portion of the bloodand the enclosing boundaries of the flow path, prevents or at leastlimits contact of the blood with these boundaries. In turn, thisconfiguration of the two fluids prevents or at least reduces theundesirable activation of factors in the blood, thereby minimizingbioincompatibilities that have been problematic in prior techniques ofblood processing.

The invention also eliminates or at least substantially reduces thefouling reactions that have been known to be a major deterrent to thecontinuous use of an extracorporeal extraction device. In particular, asthe primary transport surface in the membraneless exchange device (alsoreferred to herein as a membraneless separator) of the invention isintrinsically non-fouling, a major deterrent to long-term or continuousoperation is removed, opening the possibility to the design andconstruction of small, wearable devices or systems with the recognizedbenefits of nearly continuous blood treatment. Such a device or systemcould be very small and worn or carried by the patient (e.g., outside ofa hospital or clinic setting), and could be supplied with externalbuffer reservoirs (in a back-pack, briefcase, or from a reservoirlocated in the home, located at the place of work, etc.). Further,because fouling would be reduced, and sustained operation at low bloodflows over long times would be allowed, such anticoagulation as might berequired is likely to have an effect confined to the extracorporealcircuit. As understood by those skilled in the art, avoiding systemicanticoagulation outside of the clinic is highly desirable.

The devices, systems and methods of the invention described herein alsohave the benefit of being capable of diffusing various blood componentshaving different sizes. In particular, the flow of blood and a misciblefluid with which it is in contact can be controlled for the purpose ofachieving the desired separation of components (e.g., separatingmolecules of low molecular weight only). For example, as explainedbelow, various flow conditions may be used that cause blood cells tomove away from the blood-liquid interface, thereby making it is possibleto “skim” blood in order to remove substantial amounts of plasma,without cells.

As also discussed below, membraneless contact of a thin layer of bloodwith a sheathing fluid according to the present invention may be used tocause high rates of exchange per unit area of blood-sheathing fluidcontact for all solutes, but with a discrimination among free (unbound)solutes that is less than the square-root of the ratio of theirdiffusion coefficients. Moreover, while high exchange rates (e.g., oftoxic substances) are often desirable, indiscriminate transport is not.Therefore, according to the principles of the present invention, amembraneless exchange device as described herein is used in conjunctionwith at least one secondary processor (e.g., a membrane device or othertype of separator) in order to restrict the removal of desirablesubstances and effect the removal of undesirable substances from blood.The efficiency of such a secondary separator is greatly increased by theuse of the primary separator that is capable of delivering cell-depleted(or cell-free) fractions of blood to it. Therefore, according to anotheraspect of this invention, transport of molecular components of blood tothe sheathing fluid may be indiscriminate. The sheathing fluid, carryingboth those molecular components which it is, and is not, desirable toremove from blood, is provided to the secondary separator, such that thefluid entering the secondary separator is substantially cell-free. Thesecondary separator, meanwhile, regulates the operation of themembraneless separator through the composition of the recycle streamthat it returns (directly or indirectly) to the sheath fluid inlets ofthe membraneless separator. According to the principles of the presentinvention, moreover, a membrane-based secondary separator used in thismanner is able to achieve much higher separation velocities becauseconcentration polarization (i.e., the accumulation of material rejectedby the secondary separator on the upstream side of the separator) islimited to proteins and does not involve cells. Moreover, because cellswould be retained in the primary separator (i.e., the membranelessexchange device), they would see artificial material only on its conduitsurfaces, not on its liquid-liquid contact area, whencebioincompatibilities should be much reduced. As such, it should beunderstood that the need for anticoagulation may be greatly reduced oreliminated.

Further features of the invention, its nature and various advantages,will be more apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the velocity profile of a core stream of blood sheathed onboth of its sides by a dialysate fluid calculated for blood with aviscosity assumed twice that of the dialysate fluid and with acenterline velocity of 5 cm/sec.

FIG. 2 shows a plot using Loschmidt's formula of 1870, where each fluidlayer has the same thickness.

FIG. 3 shows a simplified view of a membraneless separator constructedin accordance with the principles of the present invention.

FIG. 4 shows membraneless separator used for the purpose ofplasmapheresis in accordance with the principles of the presentinvention.

FIG. 5 shows the image of a portion of the membraneless separator ofFIG. 5 while plasma is being skimmed from blood, as obtained by using aCCD camera.

FIG. 6 shows a simplified block diagram of a system including amembraneless separator and a secondary separator in accordance with theprinciples of the present invention.

FIG. 7 shows a more detailed view of a system including primary andsecondary separators in accordance with the principles of the presentinvention.

FIG. 8 shows the configuration of a system subdivided into three unitsin accordance with the principles of the present invention.

FIG. 9 shows the routing of fluids between separate units in accordancewith the principles of the present invention.

DETAILED DESCRIPTION

According to one aspect of the invention, a membraneless exchange devicefor extracting components from a sample fluid is described whichincludes first, second and third inlet channels, first, second and thirdexit channels and a microfluidic extraction channel connected to thefirst, second and third inlet channels and the first, second and thirdexit channels. Moreover, laminar flows of a first extractor fluid, thesample fluid, and a second extractor fluid are established inside theextraction channel, and sheathing of the sample fluid by the first andsecond extractor fluids substantially limits contact between the samplefluid and the surfaces of the extraction channel. In one embodiment ofthe device, at least 90% of the sample fluid is sheathed by the firstand second extractor fluids. In other embodiments, 95% of the samplefluid is sheathed. In yet other embodiments, at least a portion of thesample fluid exits the device with the first extractor fluid through thefirst exit channel, and advective transport of molecules within saidextraction channel is substantially nonexistent. The composition of thefirst extractor fluid, moreover, is substantially the same as thecomposition of the second extractor fluid is various embodiments. Inother preferred embodiments, the sample fluid flow is between the firstand second extractor fluid flows. Moreover, a first diverter is formedfrom a portion of the first exit channel and a portion of the secondexit channel, while a second diverter is formed from a portion of thesecond exit channel and a portion of the third exit channel. It shouldalso be understood that the device may include a first interface formedbetween the first extractor fluid flow and the sample fluid flow that isaligned with at least a portion of the first diverter, and may alsoinclude a second interface formed between the second extractor fluidflow and the sample fluid flow that is aligned with at least a portionof the second diverter. In various embodiments of the invention,moreover, the sample fluid is blood fluid, in which case it iscontemplated that the components extracted from the sample fluid arenon-cellular components of the blood fluid. Additionally, the device mayuse a first pump for controlling the flow of extractor fluid in theextraction channel, and may use a second pump for controlling the flowof sample fluid in the extraction channel. When a first pump is used, itmay be an injection pump that controls the flow of extractor fluid intothe extraction channel, and a withdrawal pump may be used that controlsthe flow of extractor fluid out of the extraction channel. In variousembodiments, additionally, a source of extractor fluid is connected tosaid first inlet channel and a source of sample fluid connected to saidsecond inlet channel. It will be understood that the source of samplefluid can be, for example, a human being. In preferred embodiments,moreover, the extraction channel of the device according to theinvention has a height of less than 600 μm, and has a width-to-heightratio of at least ten. The device may also be used in a system forextracting components from a sample fluid, where the system alsoincludes a secondary processor that receives the first extractor fluid,the second extractor fluid and at least some of the components of thesample fluid upon exiting the extraction channel. It will be understoodthat the secondary processor may be, for example, a membrane device or asorption device.

According to another embodiment of the present invention, a system forperforming hemodialysis is provided which includes a membranelessexchange device including first and second dialysate inlet channels,blood inlet and exit channels, first and second dialysate exit channelsand a microfluidic dialysis channel connected to the first and seconddialysate inlet and outlet channels and the blood inlet and exitchannels. Moreover, laminar flows of a first dialysate fluid, bloodfluid, and a second dialysate fluid are established in order inside thedialysis channel, and at least some of the components of the blood fluidexits the device through the first and second dialysate exit channels.Additionally, according to the invention, a secondary processor receivesthe dialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels. In various embodiments, the secondary processor filters thedialysate fluid and the at least some of the components of the bloodfluid exiting the device through the first and second dialysate exitchannels, and returns the filtered fluid to the first and seconddialysate inlet channels. In certain preferred embodiments, thesecomponents of the blood fluid are substantially non-cellular componentsof the blood fluid. In other embodiments, sheathing of the blood fluidby the first and second dialysate fluids substantially limits contactbetween the blood fluid and the surfaces of the dialysis channel.Moreover, the secondary processor may be a membrane device, or may be asorption device, for example. It will also be understood that thecomposition of the first dialysis fluid may be substantially the same asthe composition of the second dialysis fluid. According to other aspectsof the invention, meanwhile, a first diverter is formed from a portionof the first dialysate exit channel and a portion of the blood exitchannel, and a second diverter is formed from a portion of the bloodexit channel and a portion of the second dialysate exit channel. A firstpump for controlling the flow of dialysate fluid in the dialysis channeland a second pump for controlling the flow of blood fluid in thedialysis channel may also be used in accordance with the principles ofthe present invention. According to several embodiments, the interfacebetween the first dialysate fluid and the blood fluid is varied byadjusting the velocities of the laminar flows of the first dialysatefluid and the blood fluid. In other embodiments, the interface betweenthe blood fluid and the second dialysate fluid is varied by adjustingthe velocities of the laminar flows of the blood fluid and the seconddialysate fluid. A reservoir for storing a viscosity agent may also beused in the system, where the viscosity agent is mixed with the firstand second dialysate fluid to alter the viscosity of the first andsecond dialysate fluid. Additionally, a detector for detecting apresence of an undesired blood component within the dialysate fluid uponexiting the dialysis chamber may be used. In this case, for example, thedetector is a photo detector. According to another aspect of theinvention, a first pump for controlling the flow of dialysate fluid inthe dialysis channel is adjusted based on said detected presence of anundesired blood component within said dialysate fluid. Moreover, forexample, the velocities of the laminar flows of the first dialysatefluid, the blood fluid and the second dialysate fluid are adjusted basedon the detected presence of an undesired blood component within thefirst and second dialysate fluids according to the invention.Additionally, according to the invention, the first and second dialysatefluids may include at least one of the following: a hyper osmolarsolution, a solution high in glucose content, or a polyelectrolyeosmotic agent.

In yet another embodiment of the present invention, a method forextracting components from a sample fluid is provides which includesestablishing laminar flows of a first extractor fluid, the sample fluidand a second extractor fluid inside a microfluidic extraction channel.Sheathing of the sample fluid by the first and second extractor fluids,moreover, substantially limits contact between the sample fluid and thesurfaces of the extraction channel. The method further includeswithdrawing the first extractor fluid, the sample fluid and the secondextractor fluid from the extraction channel such that at least a portionof the sample fluid is removed together with the first extractor fluidand the second extractor fluid and apart from the remainder of thesample fluid. According to the invention, moreover, establishing laminarflows includes providing first, second and third inlet channels andproviding first, second and third exit channels. Additionally, forexample, the method includes providing the first and second extractorfluids and the at least a portion of the sample fluid to a secondaryprocessor.

A method for performing hemodialysis is also provided which includesestablishing laminar flows of a first dialysate fluid, blood fluid and asecond dialysate fluid inside a microfluidic extraction channel,withdrawing the first dialysate fluid, the blood fluid and the seconddialysate fluid from the extraction channel such that at least some ofthe components of the blood fluid are removed together with the firstdialysate fluid and the second dialysate fluid and apart from theremainder of the blood fluid, and providing the first and seconddialysate fluids and the at least some of the components of the bloodfluid to a secondary processor. In various embodiments, the method alsoincludes using the secondary processor to filter the first and seconddialysate fluids and the at least some of the components of the bloodfluid, as well as returning the filtered fluid from the secondaryprocessor to the extraction channel. In yet other embodiments, themethod includes sheathing the blood fluid by the first and seconddialysate fluids to substantially limit the contact between the bloodfluid and the surfaces of the dialysis channel.

Referring to FIG. 1, calculated for blood with a viscosity assumed twicethat of the sheathing fluid and with a centerline velocity of 5 cm/sec,a flow path length of 10 cm would result in a contact time of slightlylonger than 2 sec. The steady contact of two moving liquids for anexposure time determined by the length of their contact area divided bytheir interfacial velocity (τ=L/v) is highly analogous to the suddenexposure of one volume of stagnant fluid to another for a specifiedtime. Thus, what happens to the flowing fluids along their shared flowpath is comparable to what would happen to two stagnant fluids overtheir exposure time to each other. The stagnant fluid problem was solvedby Loschmidt in 1870.

$E = {\frac{1}{2} - {\frac{4}{\pi^{2}}{\sum\limits_{0}^{\infty}{\frac{1}{\left( {{2n} + 1} \right)^{2}}{\exp\left\lbrack {{- \left( {{2n} + 1} \right)^{2}}\left( \frac{\pi}{2B} \right)^{2}{Dt}} \right\rbrack}}}}}$for which the zeroth order term,

${E = {\frac{1}{2} - {\frac{4}{\pi^{2}}{\exp\left( {{- \left\lbrack \frac{\pi}{2B} \right\rbrack^{2}}{Dt}} \right)}}}},$suffices when

${\left( \frac{\pi}{2B} \right)^{2}{Dt}} > {0.7.}$

This formula greatly simplifies the estimation of how much mass can betransferred between fluids in a membraneless system. In particular, thisformula provides an approximation of the extraction E of a componentwith a diffusion coefficient D when two liquids flow side-by-side andremain in contact for an interval of time, t.

FIG. 2, meanwhile, shows a plot using a version of Loschmidt's formula,where each fluid layer has the same thickness B (i.e., B is thehalf-thickness of the sheathed layer of sample fluid). The situationshown in the plot of FIG. 2 can be interpreted as a blood layer, ofthickness B, contacting a layer of sheathing fluid (i.e., extractorfluid). The sheathing layer is presumed to be at zero concentration andE is the fraction of material in the blood layer that is extracted in atime t, where D is the diffusion coefficient of the extracted substance.If a layer of thickness twice B is bounded on both sides by fluid layersof thickness B, the formula still applies, as written. As indicated bythis formula, E cannot exceed ½ since the prescription of concurrentflow allows, at best, the two fluids to come to equilibrium.

For example, if one prescribes 90% of maximum extraction (E=0.45), theratio Dt/B² must be approximately 0.86. Any combination of diffusivity,layer thickness, and exposure time that produces this value will producethe same extraction. Moreover, it can be shown that the necessary area(2 LW) to achieve this extraction equals 0.86 BQ/D, where Q is the blood(and sheath fluid) flow rate. Thus, for urea (D=10⁻⁵ cm²/sec) at a bloodflow rate of 0.3 cm³/sec, the required area is 2.57 B 10⁴ cm². If B istaken to be 100 μm, the required area is 257 cm². This flow correspondsto what might be needed in a wearable artificial kidney. If, instead, aconventional flow of 5 cm³/sec were used, the required area would be4300 cm². Thinner films, moreover, would require less area but wouldresult in higher shear rates and pressure gradients. In terms ofextraction, any combination of length L and width W that produces therequisite area is equivalent. (If one assumes D for albumin to be 5 10⁻⁷cm²/sec, its extraction would be 0.116, 26% of that for urea,unchangeable at this extraction level for urea).

It should be noted that use of the Loschmidt formula with flowingsystems introduces an incongruity that prevents precise estimation ofmass transfer rates and clearances, given that it presumes that bothfluids are moving at uniform velocity. In particular, it provides anexcellent approximation for the sheathed fluid (blood), but ignores thenearly linear decay in velocity with distance from the interface in thesheathing fluid. Nevertheless, the Loschmidt formula is adequate fordesign purposes when the sheathing layer has a total thickness (2B) thatis twice that of its half of the blood layer (as shown in FIG. 1), andthus a rate of flow nearly equal to its half of the central stream.

The shear-induced self-diffusion coefficient of cells, meanwhile, can beestimated by using the expression of Leighton and Acrivos (1987) forconcentrated suspensions: D_(particle)∝φ²a²{dot over (γ)}², where φ isthe particle volume fraction, a is the particle radius, and {dot over(γ)} is the shear rate. Then, the characteristic displacement of a cellcan be expressed as Δy∝√{square root over (D_(particle)t)}. Choosingrepresentative values for the layered flow system such that the cellvolume fraction φ≅0.45/2=0.225, the average radius a of the red bloodcell ≅2.5 μm, and the average shear rate {dot over (γ)} over the bloodlayer ≅3 to 28 s⁻¹ (based on an average velocity range of 0.5 to 5cm/s), we calculate that D_(particle)˜10⁻⁸ cm²/s, which is approximatelythree orders of magnitude smaller than the typical diffusion coefficientof small solutes. Based on this value of the shear-induced diffusioncoefficient (and assuming 10 sec of contact between layers), it isestimated that blood cells are displaced by a characteristic distanceΔy≅3 to 9 μm from the central layer, depending on the choice of bloodvelocity and the concomitant shear rate. As explained in greater detailbelow, this low distance of cell migration away from the central layerfacilitates the removal of cell-free portions of blood by themembraneless separators described herein.

It should be noted that, according to one aspect of the presentinvention, the removing of undesirable materials from a sample fluidoccurs under conditions that prevent advective mixing of blood and thesecondary fluid. In its general usage herein, advection is used todescribe the transport of fluid elements from one region to another, andis used to distinguish disordered convection from diffusion unaided byconvection or diffusion in the presence of only ordered andunidirectional convection. The term advection is therefore used to meana form of transport within a fluid or between two contacting misciblefluid in which clumps of fluid from two different positions areeffectively interchanged. Advection, so defined, can occur in turbulentflows or in unstable laminar flows. Advective mixing, moreover, is oftenpurposefully induced by the application of a moving agitator blade to afluid. The prevention of advective mixing and the short contact timesthat lead to small areas of contact (and, in turn, to a small devicethat has a small size and a limited extracorporeal blood volume) isgreatly facilitated by the use of a microfluidic geometry. An increasein channel height raises requisite contact time and tends to reduce thestability of the sheathed flow. When total blood layer thickness is 25,50, or 100 μm, and the blood flow is 20 ml/min (as it might be with awearable artificial kidney), the interfacial area needed to cause asubstance such as urea (D=10⁻⁵ cm²/sec) to reach 90% of equilibrium is,respectively, 18, 36, and 71 cm².

As mentioned above, the devices, systems and methods of the presentinvention allow the purification of blood without the use of a membraneby contact of the blood with a miscible fluid under conditions thatprevent advective mixing. It will be clear from the detailed descriptionof various embodiments of the invention provided below that theinvention is useful in hemodialysis, for example. However, it shouldalso be noted, and understood by those skilled in the art, that thepresent invention is also useful in other situations where a samplefluid is to be purified via a diffusion mechanism against another fluid(e.g., an extractor fluid).

According to the principles of the present invention, the purificationtechniques described herein enable the flow of blood, completely orpartially surrounded by another liquid (e.g., extractor fluid) such thatthe streams are contacted in a small channel and are subsequentlyseparated at the end of the channel. The middle stream is, thus, theblood to be purified, while the surrounding stream (or streams) is theextractor fluid. This membraneless contact, or sheathing of blood withlayers of a miscible fluid, according to principles of the presentinvention, may occur along a flow path whose cross-section is eitherrectangular, preferably of great breadth and limited thickness, orcircular. The invention is not limited in this manner.

Persons skilled in the art will appreciate that the requisite transportareas, moreover, can be achieved by combinations of channel length,width, and number according to the principles of the present invention.In particular, Area=2 (top and bottom)×width×length×number of channelsstacked or otherwise arrayed in parallel. (As used herein, the term“width” refers to a dimension perpendicular to the direction of flow andparallel to the interface between the two liquids, while, as explainedabove, the term “height” refers to a dimension perpendicular to thedirection of flow and also perpendicular to the interface between thetwo fluids). It is shown herein that the competing requirements of smallheight (to avoid excessive diffusion times and in-process volumes),short length (to avoid excessive pressure drop) and practicallimitations on width of a single device, which suggests the need toarray them in parallel, side-by-side or in a stack can be satisfied inpractical microfluidic devices.

FIG. 3 shows a simplified view of a membraneless separator 300fabricated in flat-sheet configuration in accordance with the principlesof the present invention. According to one embodiment of the presentinvention, three flat strips of copper foil, each three centimeterswide, four centimeters long and 100 microns thick, are soldered in theirmid-sections to form extraction channel 302. The ends (one centimeter)of the outer pieces are bent 30 degrees outward to form three separateinlet channels 304, 306 and 308 and three corresponding exit channels310, 312 and 314 as shown in FIG. 3. According to the invention, thepieces are then coated with a mold release agent, and the channel isthen placed in a Petri dish. At this time, an amount of PDMSprecursor/curing agent mixture (10:1 ratio), sufficient to form a twocentimeter-thick polymer layer after curing, is poured into the dish.After curing, the foil assembly is easily released from the PDMSreplica, and the replica is sandwiched between two partially cured flatpieces of PDMS and annealed to form a well-sealed channel. Finally,slight vacuum is applied during the annealing to remove air bubblestrapped between the flow channel module and the flat pieces, and thesealed separator 300 is then ready for use (preferably after the chip isrinsed with ethanol and with de-ionized water, and then dried withcompressed nitrogen gas). A flat piece of PDMS which served as a coverto seal the chip by adhesion is also preferably cleaned and dried in thesame manner.

It will be understood that the particular fabrication process describedabove is for purposes of illustration only. For example, the dimensionsof membraneless separator 300 may be altered without departing from thescope of the present invention. Additionally, for example, it will beunderstood that the invention is not limited to the use of copper foil,and that other fabrication processes not described may also be employed.

FIG. 4 shows a membraneless separator 400 according to the principles ofthe present invention. Similar to separator 300 described above,separator 400 includes an extraction channel 402, three separate inletchannels 404, 406 and 408 and three corresponding exit channels 410, 412and 414. As also shown in FIG. 4, a first diverter 416 is formed fromportions of exit channels 410 and 412, while a second diverter 418 isformed from portions of exit channels 412 and 414. It will beunderstood, however, that the invention is not limited by the number ofexit channels (or inlet channels) that are used, nor is the inventionlimited by the number of diverters formed therefrom.

As illustrated in FIG. 4, membraneless separator 400 can be used as aplasmapheresis device in accordance with the principles of the presentinvention. For example, as shown in FIG. 4, plasma from the bloodentering extraction channel 402 through inlet channel 406 is skimmed andexits with sheath fluid through exit channels 410 and 414. This processof skimming is explained in greater detail below in connection with FIG.7.

FIG. 5, meanwhile, shows an image of the right-most portion of separator400 shown in FIG. 4, as obtained by using a CCD camera (Sensys0401E,Roper Scientific). In particular, the image of FIG. 5 illustrates plasmabeing skimmed from blood according to the principles of the presentinvention. As shown in FIG. 5, a portion of the blood 501 providedthrough inlet channel 402 (not shown) exits through exit channel 405.Moreover, while cellular components of blood 501 migrate to the center(as explained below in connection with FIG. 7), cell-depleted (orcell-free) fractions of blood 501 such as plasma 502 and 503 combinewith sheath fluid 504 and 505 to exit extraction channel 400 throughexit channels 404 and 406, respectively.

It will be understood by persons skilled in the art that a membranelessseparator as described herein is not intended to, nor could it, offersufficient discrimination between the substances it is intended toremove and those it is intended to leave behind. Accordingly, forexample, membraneless separators as described above will only functionby themselves in the exceptional circumstance that all the components ofplasma are to be removed. For example, a membraneless separator may beused alone when the removal of plasma, usually not in its entirety butwithout discrimination among its components, is to be removed, and thecellular components of blood are to be left behind.

In all other circumstances, according to the principles of the presentinvention, a membraneless separator will operate in conjunction with asecondary separator that receives the sheath fluid and, optionally, acell-depleted (or cell-free) part of the bloodstream. For example, toprevent the removal of macromolecules, the secondary separator can beused to generate a stream rich in macromolecules and free of smallmetabolite molecules and middle molecules that is recycled in sheathfluid to the membraneless separator. Thus, according to the invention,the secondary separator regulates the operation of the membranelessseparator through the composition of the recycle stream that it returnsto the inlets for sheath fluid of the membraneless separator (as shownin FIG. 6 and described in greater detail below). It should beunderstood that the secondary separator may incorporate a variety ofmeans to remove solutes whose extraction removal from the circulation(i.e., the recycle stream) is desired, and that the invention is notlimited in this manner.

One substance whose transport (i.e., removal from blood being processed)is typically undesirable is albumin. In each pass through an exchangedevice according to the invention, for example, albumin would be removedat more than ¼th the rate of small solutes, and albumin (which isconfined to the blood space of an animal) would undergo perhaps 10 timesas many passes as would urea which is distributed throughout the totalbody water reservoir. Thus, the fractional removal of albumin, eventhough its inherent diffusivity is smaller, would exceed the fractionalremoval of urea. According to the principles of the present invention,therefore, a secondary separator (e.g., a membrane device that permitsextraction of urea and water but not albumin) may be used to recyclealbumin to the blood. In particular, the sheath fluid received from therecycle stream will be depleted of urea and water, but will be rich inalbumin. Thus, the composition of this stream will recruit the furtherextraction of urea and water but will not recruit further extraction ofalbumin, given that the difference in albumin concentration between theblood being processed and the sheath fluid will have disappeared.

It will be understood that an important specification of how themembraneless separator operates is the difference between the inlet flowrate and the outlet flow rate of the sheath fluid. For example, whenthese flows are equal and urea and water are removed by the secondaryseparator, there will be, at first, an insufficient transfer of waterfrom blood to the sheath fluid to keep up with water removal in thesecondary separator. Thus the concentration of proteins, includingalbumin, will rise in the recycle stream. When this concentration hasreached a sufficiently high level, water transfer will be enhanced by adifference in protein osmotic (oncotic) pressure between the blood andthe sheath fluid. Thus, the membraneless separator will balance itsperformance to that of the secondary separator. On the other hand, ifthe rate of withdrawal of sheath fluid is greater than its rate ofsupply, sufficient water may be sent to the secondary separator to keepup with its rate of water removal, but protein concentration will riseagain until a concentration difference exists in the membranelessseparator between the sheath fluid and the blood, causing a diffusion ofprotein back into the bloodstream. Once again, the membranelessseparator will balance its performance to that of the secondaryseparator.

For example, when the principal goal of the treatment is the removal ofhighly diffusible (in general, low molecular weight) molecules, assuminga flow of 20 ml/min flow, the contact area in the membraneless separatorwill be in the range about 17 to 71 cm². When the principal goal of thetreatment is the removal of slowly diffusible molecules (e.g., proteinsand especially immunoglobulins), the contact area in the membranelessseparator will be larger, in the range of approximately 1,700 to 7,100cm² (assuming a flow of 20 ml/min), and the secondary separator will beconfigured to remove these molecules and to recycle smaller molecules(unless their simultaneous removal is desired).

FIG. 6 shows a simplified block diagram of a system 600 includingmembraneless separator 602 and secondary separator 604 in accordancewith the principles of the present invention. Although not shown indetail, it will be understood that membraneless separator 602 may besimilar to those separators shown in FIGS. 3 and 4 and described above,for example.

According to the principles of the present invention, blood that is toundergo processing is provided to (and removed from) membranelessseparator 602. Meanwhile, sheathing fluid that is recycled by secondaryseparator 604 is also provided to (and removed from) membranelessseparator 602. As also shown in FIG. 6, whenever secondary separator 604transfers solutes to a second fluid (e.g., dialysate), fresh dialysateconnection 606 and waste dialysate connection 608 may be used forproviding fresh and waste dialysate streams, respectively. It will beunderstood that shunting of fresh fluid directly to the blood stream, asrepresented by dashed line 610, is also a possibility (but notmandatory). In general, FIG. 6 makes the role of membraneless separator602 clear: to equilibrate solutes of interest with the sheathing fluidwithout transfer of cells.

It will be understood that secondary separator 604 may use any of manyavailable separation principles known to those skilled in the art,including ultrafiltration and sorption using a wide range of sorbentstargeted to particular small and large molecules, chemical reaction, andprecipitation. Plasma diafiltration (a variant of hemodiafiltration),for example, may also be used according to the principles of the presentinvention. The following international publications which refer tohemodiafilters are incorporated by reference herein: WO 02/062454(Application No. PCT/US02/03741), WO 02/45813 (Application No. PCTUS01/47211), and WO 02/36246 (Application No. PCT/US01/45369). Accordingto additional embodiments of the present invention, moreover, whenlow-molecular weight solutes are to be removed by plasma diafiltration,a stream of sterile buffer is added to the blood to allow a greatervolume of fluid, with its accompanying small molecules, to pass throughthe diafiltration membrane. In conventional diafiltration, this volumemay be added before or after the diafilter. In this invention, however,it is advantageous to add it either to the bloodstream or the recyclefluid from the secondary separator 604, which is the primary source ofsheath fluid.

A more detailed view of a system 700 which includes membranelessseparator 702 and secondary separator 704 in accordance with theprinciples of the present invention is shown in FIG. 7. As shown in FIG.7, separator 702 includes extraction channel 706, inlet channels 708,710 and 712 and exit channels 714, 716 and 718.

According to the principles of the present invention, system 700 alsoincludes blood supply 720, and a plurality of pumps 722, 724 and 726(which may be either manually or automatically operated, such as byusing detection and regulation techniques described below). As shown inFIG. 7, blood supply 720 provides blood to be processed to membranelessseparator 702 through blood inlet channel 710. It will be understoodthat blood supply 720 may be a living person or other animal, forexample, or may be a blood reservoir. Blood withdrawal pump 722,meanwhile, is responsible for removing blood from separator 702 throughblood exit channel 716.

As illustrated by FIG. 7, the flow of sheath fluid (or extractor fluid)into separator 702, through sheath inlet channels 708 and 712, iscontrolled by sheath fluid injection pump 724 (which preferably providessheath fluid in equal parts to channels 708 and 712). The flow of sheathfluid out of separator 702, through sheath exit channels 714 and 718,meanwhile, is controlled by sheath fluid withdrawal pump 726 (whichpreferably draws equal amounts of sheath fluid out of channels 714 and718). According to preferred embodiments of the present invention, pump724 is a two-chamber pump that provides sheath fluid at equal velocities(and with substantially similar composition) to both inlet channels 708and 712, while pump 726 is a two-chamber pump that removes sheath fluidfrom exit channels 714 and 718 at equal velocities. Moreover, it is alsocontemplated that pump 724 be replaced by two pumps (not shown) forseparately providing sheath fluid to inlet channels 708 and 712, inwhich case the composition of the sheath fluid entering inlet channel708 may be substantially similar to, or different from, the sheath fluidentering inlet channel 712. Similarly, two pumps (not shown) can be usedin place of pump 726 for the purpose of separately withdrawing sheathfluid from exit channels 714 and 718. It is also contemplated that, inother embodiments of the present invention, sheath fluid enteringthrough inlet channel 708 and exiting through exit channel 714 flows ata different velocity than the sheath fluid entering through inletchannel 712 and exiting through exit channel 718. It will be understoodthat the invention is not limited by the particular usage of pumps orsheath velocities described herein in connection with the description ofFIG. 7.

As explained above, a membraneless separator according to the inventionalso needs one or more diverters to operate. Thus, according to theprinciples of the present invention, a first diverter 727 is formed froma portion of sheath exit channel 714 and a portion of blood exit channel716. Moreover, a second diverter 728 is formed using a portion of bloodexit channel 716 and a portion of sheath exit channel 718. It will beunderstood that, in embodiments of the present invention using more thantwo layers of sheath fluid, addition diverters will be used.

In certain preferred embodiments of the invention, the sheath fluidprovided to separator 702 (from separator 704 and/or optional sheathfluid reservoir 730) by sheath fluid injection pump 724 occupiesapproximately ⅔ of the cross-section of extraction channel 706, withblood from blood supply 720 in the middle ⅓. In this manner, each halfof the blood layer in extraction channel 706 is “serviced” by one of thesheathing layers, and the sheathing layers are traveling at an averagevelocity that is approximately half that of the blood (even though theinterfacial velocities of the blood and sheathing fluids are equal).Thus, the volume of blood and the volume of sheathing fluid that passthrough the unit in a given period of time are approximately equal.Although the invention is not limited in this manner, it should be notedthat, in the configurations described here, efficiency drops when thevolumetric flows of the two fluids (i.e., blood and sheath fluid) arevery different from each other.

In order to cause the separation (or skimming) of all or part of thecell-depleted component of the blood being processed, according tovarious embodiments of the present invention, the inlet and exit flowsof the sheath fluid are controlled (via pumps 724 and 726, respectively)such that more sheath fluid is withdrawn from separator 702 than isprovided thereto. For example, it is possible to skim 10% of the bloodflow by running sheath fluid withdrawal pump 726 at a rate that is 10%higher than the rate of sheath fluid injection pump 724. It will beappreciated that, when this is done, the blood efflux rate is determinedand need not be controlled, as it should naturally have an outflow thatis 90% of the inflow.

As explained above, when indiscriminate plasma removal is not desired,the plasma that is skimmed from the blood using membraneless separator702 is processed by secondary separator 704, which regulates theoperation of separator 702 through the composition of the recycle streamthat it returns to sheath inlets channels 708 and 712 (i.e., a recyclestream is used to limit transport of blood components for whichextraction is not desirable). According to the principles of the presentinvention, a substantial benefit arises because secondary separator 704,whether membraneless or not, is able to achieve high filtrationvelocities due to the fact that concentration polarization is limited toproteins and does not involve cells. Moreover, because cells areretained in the membraneless separator 702, they would see artificialmaterial only on its conduit surfaces, not on its liquid-liquid contactarea, with the result being a reduction in bioincompatibilities and areduced (or eliminated) need for anticoagulation. Additionally, becausethe primary transport surface in the system is intrinsicallynon-fouling, a major deterrent to long-term or continuous operation isremoved, opening the possibility of a wearable system with therecognized benefits of prolonged, slow exchange.

It should be understood that any operation of membraneless separator 702that allows the sheath exit flows to be larger than the correspondinginlet values will induce a convective flow from the blood stream, overand above the diffusive flow. In order to prevent such a convective flowfrom carrying blood cells with it (as would be the case if thedistribution of cells in the blood stream was uniform), it is importantthat cellular components of the blood have migrated to the center of theblood stream in order to permit significant plasma skimming. As shouldbe appreciated by those skilled in the art, centripetal drift of cellsoccurs under a variety of flow regimes. According to the invention,therefore, various flow conditions can be used that cause blood cells tomove away from the blood-liquid interface. For example, when blood flowsin a tube below a wall shear rate (measured as the blood-flow velocitygradient perpendicular to the tube wall) of about 100 reciprocalseconds, this shear rate causes cellular components to migrate thecenter and leave the sheath as cell-free, essentially pure plasma. (SeeGoldsmith, H. L. and Spain, S., Margination of leukocytes in blood flowthrough small tubes, Microvasc. Res. 1984 March; 27(2):204-22.)

It will be appreciated that long-term stability is necessary forsatisfactory operation of the microfluidic devices described herein. Forexample, it is desirable to prevent inappropriate splitting of an exitstream which, uncorrected, could result either in loss of cells orunintended infusion of sheathing solution into the bloodstream.Moreover, the presence of blood cells in the sheath, or extractor fluidmay also be undesirable. According to another aspect of the presentinvention, therefore, on-board electronics and photonics (not shown),which are common features of chip-based microfluidic devices, may beused. In particular, such electronics or photonics could be used toregulate system 700 (i.e., to introduce flow changes) with anelectrically activated device (e.g., a piezoelectric valve) that ismounted on the same plate, or “chip,” on which separator 702 is located.

According to one embodiment of the invention, for example, very lowconcentrations of cells would be permitted and monitored (e.g., beforeor after the sheath fluid being provided to secondary separator 704) byusing any suitable detector, such as a photo detector. Anultramicroscope (a light-scattering device that is particularlysensitive to the presence of dilute particles) is one example of a photodetector which can be used. Based on this monitoring, flow correctionsthat would provide the system with long-term stability can be made whichinclude, for example, adjusting the blood-sheath fluid interface. Inparticular, by adjusting the flows to separator 702 to reposition theinterface, desired components can be retained in the blood. For example,when an excessive number of blood cells is present, the flow of bloodcould be decreased (or the flow of extractor fluid increased) in orderto shift the blood-sheath fluid interface accordingly.

Additionally, according to another aspect of the invention, on-boardelectronics can be used to protect against the type of flow imbalancesthat might cause large blood losses in one direction or massivehypervolemia in the other direction, which are naturally prevented whena membrane is present but which may occur in a membraneless device. Itwill be understood by those skilled in the art this type of detectionand regulation may also be used with in conjunction with the otherembodiments of the present invention described above.

As explained above, in all membraneless contact configurations, thefluids (e.g., blood and sheath fluid) must flow in the same direction.In particular, any flow in opposite directions would disrupt theblood-fluid interface and induce undesirable advection. Moreover, sincethe fluids must flow in the same direction, the most that can beaccomplished in one membraneless unit according to the invention is theequilibration of the sheath and blood streams (which, according toLoschmidt's formula provided above, means that if the sheathing fluid isflowed at the same rate as blood, the extraction E of a solute cannotexceed ½). In other words, if the two flows are equal, at most half ofany solute can be transferred. Moreover, while greater flows permitlarger fractions, E, of a solute to be removed, they require highercirculation rates to the secondary separator and thus force processingof solutes at lower concentrations, which is generally undesirable.Therefore, it is generally desirable for these flows to be nearly equal,within at least a factor of 2 or 3.

This limitation on extraction can be largely overcome, however, by theconfigurations shown in FIGS. 8 and 9 and described below which achievethe effect of opposing flows (counterflow) by the juxtapositions ofmodules. In particular, low extraction efficiency can be overcome bymore sophisticated layouts of a microfluidic system such that flows areconcurrent in each unit of the system, but the overall flow approachescountercurrency in pattern and efficiency.

According to the invention, subdivision of a given, desired contact areainto n units each connected to the other in a countercurrent manner,even though the flow within them is concurrent, is used to allowextraction efficiency to rise according to the formula provided above.Thus, if an area were divided into four units, for example, and each hadan extraction efficiency of 50%, the composite unit would have anefficiency of 0.8 or 80%.

FIG. 8 shows the configuration of a system 800 according to theinvention in which the total area of contact is partitioned into threesub-units 802, 804 and 806 (i.e., n=3). In operation, blood to beprocessed is first provided to sub-unit 802, then passes throughsub-unit 804, and finally, exits out of sub-unit 806. The sheath fluidto be used in system 800, on the other hand, is first provided tosub-unit 806 (at this point, the sheath fluid has no blood components).The sheath fluid exiting sub-unit 806 is next provided to sub-unit 804,and after exiting sub-unit 804, is provided to sub-unit 802. Thus,assuming each unit has an extraction efficiency of 50%, the overallextraction efficiency of the composite unit, E_(O), is equal to 0.75 or75%. Accordingly, it becomes possible, at equal flows, to remove 75%rather than only 50% of the solute of interest. It will be understoodthat the extraction efficiency approaches 1.0 or 100% as the number ofsmall units approaches infinity. Persons skilled in the art willappreciate that, although not shown, the sheath fluid exiting sub-unit802 may be provided to a secondary separator as described above.Moreover, while three sub-units 802, 804 and 806 are shown in FIG. 8, itwill be understood that any number of sub-units (e.g., 2, 4, 5, etc.)may be used in system 800, all of which may be easily introduced on amaster chip fabricated according to well known techniques for thegeneral fabrication of microfluidic devices.

FIG. 9 shows another example of a system 900 using sub-units accordingto the principles of the present invention. In particular, FIG. 9 showstwo flow patterns 902 and 904 that would be superimposed on each otherin a single cartridge. For example, the top could represent blood, whilethe bottom could represent an extractor fluid (e.g., dialysate). Asshown in FIG. 9, sheath fluid flows through sub-unit 906 prior toflowing through sub-unit 908. In this manner, with sufficient contactarea, the fraction of material in the blood layer that is extracted willbe equal to ⅔ or 67%.

Persons skilled in the art will appreciate that many differentfabrication techniques can be used in accordance with the principles ofthe present invention. In recent years, controlled fluid movement andtransport among fluids has been achieved in very small channels and atvery low rates of flow largely for the purpose of assaying the contentsof a minute fluid sample in order to determine, for example, thecatabolite concentration in the blood. These devices have been enabledby recently developed microfabrication methodologies. The Holy Grail hasbeen the development of a “Lab on a Chip,” in which several sequentialanalytical processes are conducted on a single chip that may be, forexample, one square centimeter in area. Transport of a chemical orbiochemical sample from one process to another and on and off the chipitself requires fluid handling capabilities, and thus, this enablingtechnology is commonly called “microfluidics.” Microfluidics isessential for nearly all on-chip applications. The synthesis ofchemicals in microfluidic geometries is an application that is perhapscloser in concept to the scope of this disclosure because of the need toprocess a relatively larger amount of fluid. Synthesis includes,perforce, the separations needed between the steps of a chemicalreaction sequence. While the aims of synthesizers differ from ours, andembrace some issues that we do not now see as pertinent, all of thiswork, reported and emergent, is of interest. Specifically, the presentinvention embraces some of the fabrication techniques and experimentalmethods developed for the fabrication and characterization ofmicrofluidic device structures, to define upwardly scalable transport toand from blood.

According to the invention, moreover, microchannel structures for flowexperiments may be formed by a rapid-prototyping technique. For example,the required structures may be realized in PDMS (silicone) resin byreplica-molding from master structures created in thick negative photoresist (SU-8) by optical lithography. Commercially available, standardgrade mixtures of EPON SU-8 photo resist, SU-8-5 (52% solids), SU-8-25(63% solids), SU-8 50 (69% solids) and SU-8 100 (73% solids), forexample, may be spun onto Si wafer substrates at a speed of rotationthat depended on the film thickness needed, yielding films that were 10to 300 μm thick. For example, SU-8 50 spun at 1100 rpm yields a 100 μmfilm. Prior to exposure, moreover, the spun layer is preferably baked ona precisely leveled hot plate at 95° C. for a time that is dictated bythe film thickness (ranging from minutes to hours). These samples arethen allowed to cool before further processing. Post-bake exposure,meanwhile, can be done using a direct laser writing system. Thephotolithographic setup consists of an Ar-ion laser (wavelength λ=350nm), focusing optics, and a computer controlled sample stage. Themovement of the stage along all three axes (x, y, z) is achieved bystepping motors. Desired master patterns were created by translating thesamples underneath the focused laser beam to expose the outline, andthen scanning across the interior so that the intended micro channel wasfully exposed. Dynamical focus correction or the sample tilt withrespect to the scanning laser beam was the done by on-the-flyadjustments of the distance between the focusing lens and the samplestage. In a preferred embodiment, this exposure is carried out at 95° C.for 15 min. Development, meanwhile, can be carried out in a commercialSU8 developer, again for a time based on film thickness (with the samplebeing lightly stirred during development). Patterns created in SU-8,meanwhile, are used as molding masters for replication in PDMS. PDMS isprepared from a mixture of PDMS precursor and curing agent (Sylgard 184kit, Dow Corning) in a 10:1 ratio by weight. Before curing, the mixtureis placed in vacuum to evacuate bubbles formed during mixing. It is thenpoured over the SU-8 master, which had been previously coated with athin layer (˜50 nm) of chromium to improve the release of the PDMScasting, after curing. Curing is done at 70° C. for approximately twelvehours. Once the SU-8 film is spun, pre-baked and cooled as describedabove, a Karl Zeiss MJP3P Contact Mask Aligner can be used for exposure,together with standard chromium masks or transparency masks depending onthe resolution required. The films are then post-baked, and developed inthe manner outlined in the previous section. The same pattern transfertechnique is used to produce PDMS replicas.

It is apparent to those skilled in the art that many advantages may beprovided in the various embodiments of the present invention describedabove. For example, the devices, systems and methods according to theprinciples of the present invention are capable of diffusing variousblood components having different sizes, including ‘small’ molecules,‘middle’ molecules, macromolecules, macromolecular aggregates, andcells, from a blood sample to the extractor fluid. This ability isparticularly important considering the fact that different treatmentsrequire the removal of different sized particles. For example, indialysis, one may desire to remove molecules of low molecular weight,while in the treatment of acute liver failure, both small andintermediate-sized molecules are to be removed. In therapeuticapheresis, meanwhile, one generally wishes to remove selected proteinmacromolecules (e.g., immunoglobulins), while in the treatments forfulminating sepsis, it is toxins of intermediate molecular weight thatone generally desires to remove. On the other hand, in proposedanti-viral treatments, one wishes to remove free viral particles, whilein the treatment of congestive heart failure, one simply wishes toremove water.

It should also be apparent that a device or system according to theinvention may be used to process the blood of a single individual forthe purpose of treating any of a large number of disease states. Forexample, therapies according to the invention may be used in thetreatment of acute renal failure, acute liver failure, high antibodylevels in myasthenia gravis and other autoimmune diseases. Additionaluses include, for example, the removal by either precipitation orsorption of LDL in homozygous hyperlipidemia, in addition to the removalof malignant sepsis or fluid in cases of congestive heart failure, forexample. The invention may also be used to aid in the reduction of viralburdens in AIDS patients, as well as for treatment of patients requiringother types of blood purification. Patients with diabetes, patients thathave suffered a drug overdose, patients that have ingested a poison,patients suffering from renal failure, patients suffering from acute orchronic liver failure, or patients that have Myasthenia gravis, lupuserythematosis, or another autoimmune disease may also benefit from thedevices and systems of the present invention. For example, while anexchange device according to the invention is not a cure for diabetes,it can be useful in the amelioration one or more symptoms of diabetes.Moreover, the device or system of the invention could be useful inclearing the blood of IgG molecules or other molecules, which arecausative of an autoimmunity disorder. Additionally, the device orsystem of the invention can be used in acute dialysis or for extendeddialysis. One skilled in the art will also appreciate that patients (oranimals, in the case of veterinary use of the present invention)suffering from disorders, diseases and syndromes not listed herein maynonetheless be included in the patient pool intended for the device andsystem according to the invention.

Additionally, because the membraneless devices and systems describedabove have a small need for supporting machinery, and may be expected tobe much smaller, to avoid high cell concentrations and membrane contact,and to operate throughout at low rates of shear, they are especiallycompatible with cognate processes. In one embodiment, a wearable (or atleast portable) system according to the invention can run between 20 and24 hours per day at a flow rate of about 20 cc/min, for example. Thepatient could then have, for example, 4-5 hours each day without thedevice in place which could be used for personal hygiene (e.g., showersor baths), sports activities, or other activities not amenable to thesmall system being worn or used. The invention thus addresses a problemrecognized by the dialysis community (i.e., the negative side effectssuch as physical exhaustion, thirst, etc. associated with an episodicdialysis schedule), for which daily or nocturnal hemodialysis is notalways a sufficient alternative. In particular, the invention describedherein allows the patient to move about in a normal manner (e.g., go towork, school, home, etc.) while being subject to ongoing dialysis.

In addition to the treatment of various disease states, a device orsystem according to the invention may also be used for extracting bloodcomponents that are useful in treating others, as well as for purposesof studying the processes by which molecules and cells segregate anddiffuse in blood. For example, it is known to those skilled in the artthat diffusion of individual molecular species in blood may not occurindependently and may not depend on size in the simple manner dictatedby the Stokes-Einstein equation. Moreover, many solutes may partitioninto multiple forms: free, in complexes, bound to plasma protein, boundto cell-surface moieties, or as intracellular solutes. Relative to therate of diffusion of the solute, its different forms may or may not bein local equilibrium. These phenomena are likely obscured when amembrane is present because it slows and controls overall transferrates. Therefore, a membraneless device or system according to theinvention can be a useful scientific tool to study these phenomena and asystem in which rates are raised enough that partitioning may set limitson how much and how quickly a solute can be removed. A particularexample is bilirubin bound to albumin. Another example is inorganicphosphorous which exists as partially ionized salts, as two anionicforms in plasma and in several intracellular forms.

Persons skilled in the art will also appreciate that the presentinvention can be practiced by other than the described embodiments,which are presented for purposes of illustration and not of limitation,and that the present invention is limited only by the claims thatfollow.

The invention claimed is:
 1. A method of performing a blood treatment,comprising: withdrawing blood from a live patient and flowing withdrawnblood into a stack of rectangular microfluidic channels each sized andshaped, and at a flow rate, to produce a shear rate such that cellularcomponents of the blood migrate to the center of each microfluidicchannel and leave a sheath as cell-free, essentially pure plasma; eachmicrofluidic channel having an aspect ratio perpendicular to the flowdirection of at least 10, the minimum dimension being a depth and themaximum dimension being a width; withdrawing the essentially pure plasmafrom each microfluidic channel through two outlets positioned across thedepth; simultaneously withdrawing plasma-depleted blood from eachmicrofluidic channel from the middle of each microfluidic channelbetween the two outlets; combining withdrawn pure plasma flow from thestack of microfluidic channels and pumping the withdrawn pure plasma ata positive pressure through a membrane filter to ultrafilter water anduremic toxins therefrom to produce dehydrated plasma at an outlet of thefilter; returning the dehydrated plasma and the plasma-depleted blood tothe patient; performing the above operations continuously over atreatment period of time encompassing a majority of a day, the stackbeing a component of a wearable system, wherein the rate of waterremoval over the treatment period of time is sufficient to regulate thewater burden of a patient with renal failure.
 2. The method of claim 1,wherein the returning the dehydrated plasma to the patient includespumping the dehydrated plasma into the stack of microfluidic channels.3. The method of claim 2, wherein the returning the dehydrated plasma tothe patient includes shunting the dehydrated plasma directly into theblood stream of the patient.
 4. The method of claim 2, wherein thepumping the withdrawn pure plasma at a positive pressure through amembrane filter to ultrafilter water and uremic toxins from the pureplasma includes diafiltering the pure plasma.
 5. The method of claim 1,wherein the returning the dehydrated plasma to the patient includesshunting the dehydrated plasma directly into the blood stream of thepatient.
 6. The method of claim 5, wherein the pumping the withdrawnpure plasma at a positive pressure through a membrane filter toultrafilter water and uremic toxins from the pure plasma includesdiafiltering the pure plasma.
 7. The method of claim 1, wherein thepumping the withdrawn pure plasma at a positive pressure through amembrane filter to ultrafilter water and uremic toxins from the pureplasma includes diafiltering the pure plasma.
 8. The method of claim 1,further comprising regulating the flow through the microfluidic channelsto decrease the flow of blood in response to a detection of a thresholdamount of blood cells in the plasma withdrawn from each microfluidicchannel through said two outlets.
 9. The method of claim 1, wherein theaspect ratio is at least
 50. 10. The method of claim 1, wherein thechannel depth is less than 100 microns.
 11. The method of claim 1,wherein the ultrafiltering of water and uremic toxins from the pureplasma retains all the proteins in the dehydrated plasma.